function change(value) {
    if (value == "stulend") {
        $('#mainIframe').attr('src', '../html/stu_lend.html');
    }
    else if(value=="stureturn"){
        $('#mainIframe').attr('src', '../html/stu_return.html');
    }
    else if(value=="booklogoff"){
        $('#mainIframe').attr('src', '../html/book_logoff.html');
    }
    else if(value=="booksearch"){
        $('#mainIframe').attr('src', '../html/book_infor.html');
    }
    else if(value=="bookstore"){
        $('#mainIframe').attr('src', '../html/book_store.html');
    }
}